Although substantial progress has been made on understanding the mechanism of HIV-1 entry into cells, much less is known about the process of virus transmission in vivo. What structural and functional properties of the viral Env protein differentiate the transmitted virus from other variants and facilitate transmission across mucosal surfaces? In this application, we propose to analyze HIV-1 virus populations isolated from both the donor and recipient immediately following a transmission event in unique cohorts of discordant couples in Rwanda and Zambia. These studies will provide a unique opportunity to investigate the virologic determinants of heterosexual transmission specified by the variable regions of gp120 in biologically relevant viral envelope genes using samples from large, well-characterized discordant couple cohorts that represent two predominant clades (A and C) of HIV-1. Our hypothesis is that extensions and heavy glycosylation of the first two variable regions of gp120 (V1V2) confer a selective advantage to the virus during long-term infection of an individual but that these same changes are disadvantageous during viral transmission, when variants with different modifications in V1V2 have a selective advantage. Specifically we will: 1. Determine the amino acid sequence of the V1-V5 region of gp120 in virus isolated from donor and recipient at the time of acute infection in order to identify sequence variants selected during transmission; 2. Determine whether the selection of viral variants reflects the population of virus present in the genital fluids of the donor or a biological restriction of the transmission process; 3. Compare the heterogeneity of virus present in recipients at the time of acute infection by gender and by virus clade; and 4. Compare the functional properties of viral variants present in the donor and recipient at the time of acute infection. The results of these studies will enhance our understanding of the heterosexual transmission process and will yield novel information that is critical to the design and testing of globally effective vaccine candidates.